4. Discussion
The growth indexes of plants can reflect their adaptability to these substrate materials to some extent (Chi and Liu. 2016). It has been reported that the addition of biochar to sediments significantly promoted the growth of V. natans andCeratophyllum demersum (Li et al. 2020), and the addition of maifanite to sediments increased root weight by more than 93% inVallisneria spiralis and Hydrilla verticillate (Liu et al. 2020b). In this study, the addition of 4 substrates (CE, ME, VE, and VR) increased the number of leaves and root length of V. natans . Furthermore, lake sediments with high viscous strength can reduce the rooting ability of submerged plants (Handley and Davy. 2002). However, the sediments in this study were mainly clay with high viscosity, and the addition of substrate has been reported to reduce the sediment viscosity (Bai et al. 2022), which may be the main reason for the larger leaf number and better root growth of V. natans in the treatment groups than in CK group in this study. The added substrates also contain rich trace elements such as Na+, Fe2+, Mg2+, and Ca2+ (Liu et al. 2018), which can provide nutrients for the growth of submerged plants (Li et al. 2020; Wegrzyn et al. 2022). This may be another reason for the better plant growth status in the treatment groups and in CK in this study.
Our data showed that the contents of chlorophyll Chl-a and Chl-b reached the maximum value at 20-30 days post culture. At the early stage (day 10), there was no significant difference in chlorophyll content among different treatment groups (Fig.2). At the late stage (day 40), chlorophyll content in maifanite treatment group was significantly higher than that in other groups. The chlorophyll reflects the photosynthetic capacity of plants, and the chlorophyll content in maifanite group was higher than that of other groups at the late stage of plant growth, which indicated that the maifanite as substrate could improve the photosynthesis of plants, thus promoting the growth of aquatic plants.
When plants are exposed to external environmental stresses, reactive oxygen species will accumulate in the plant organelles, thus disturbing the original oxidation reduction system of plant cells, eventually damaging the structure and functions of the cell membrane system (Ahmad et al. 2016). Therefore, plants will increase one or more antioxidant enzymes such as SOD and CAT to remove reactive oxygen species so as to reduce this damage (Panda and Choudhury. 2005; Asaeda et al. 2022). Some studies have found that under the extended environmental stresses, the accumulation of lipid peroxidation product MDA increased in plant cell membrane, and its content increased with the increasing environmental stresses (Gao et al. 2012; Chen et al. 2018). The accumulation of MDA even leads to the inactivation of the enzymes related to photosynthesis, respiration, and some metabolic processes in plant cells (Song et al. 2015). In this study, MDA content was significantly different among all groups (P < 0.05). On day 10, MDA content in each treatment group reached the maximum, then gradually decreased, and finally became stable, and such a change trend occurred earlier than CK group (Fig. 3c). This might be due to the beginning of the experiment, the transplantation caused damage toV. natans, thus producing a large amount of MDA, but addition of substrates enabled plants to rapidly generate antioxidant enzymes, thus inhibiting the MDA production and promoting the adaptation of V. natans to the new environment after transplantation, eventually promoting the rapid growth of plants. Our results were consistent with one previous report that environmental stress can cause plant tissues to initate stress response mechanisms (Hao et al. 2020). Our data also showed that MDA content in vermiculite and maifanite groups on day 40 reduced by 38.2 nmol g-1 and 40.1 nmol g-1 compared with that on day 10, respectively, indicating that these two substrates could promote the adaptation of plants to adverse environment stress.
When MDA content is elevated in plants, plants will activate an antioxidant system to avoid damage caused by oxidative stress (Singh et al. 2020). The antioxidant enzyme SOD can convert O2- into O2 and H2O2 in plants, and CAT further decomposes H2O2 into non-toxic H2O2 and O2 in plants (Yin et al. 2008; Jiang et al. 2011). In the middle stage of culture (day 30), V. natans in all treatment groups produced a large amount of SOD to decompose O2-, thus avoiding the possible harm by O2-. The maifanite group exhibited the highest and the most obvious reduction of O2- in plants, and SOD content in maifanite group was always higher than that in the control group during the whole experiment period (day 1-40), indicates that addition of maifanite could better improve plants’ adaptation to the environment.
Root vitality can reflect the ability of plants to absorb water and nutrients, and plays a key role in determining plant growth status (Rewald and Meinen. 2013). In this study, the root activity was positively correlated with the plant growth indexes, and the root activity in each group reached the maximum at the middle stage of culture (day 20-30). This might be because trace elements in the substrate promoted the plant’s root activity and enzyme defense, thus weakening the damage to plants under adverse conditions and promoting the growth of plants in changing environments (Liu et al. 2020a).
In this study, the diversity of rhizosphere microorganisms and the relative abundance of Desulfobacterota and Nitrospirotawere higher than those of CK group after substrate addition onto the surface of the sediment. Han et al. (2019) also found similar results, which might be mainly due to the low oxidation reduction potential in the surface layer of sediments, thus leading to the accumulation of sulfide. Mg2+, Fe2+, and other trace elements in the form of inorganic salts in the substrate can directly participate in electron transport, oxidative stress, nitrogen fixation, and hormone synthesis (Liu et al. 2018), which promote the growth ofDesulfobacterota and Nitrospirota and the conversion of nitrogen sulfide.
In this study, the dominant bacterial phyla in all the groups were basically the same, among which Proteobacteria was the largest dominant bacterial phylum in all the groups, followed byChloroflexi , which was consistent with the report by Guan et al. (2015). Proteobacteria was considered to be the main dominant phylum in many ecosystems such as constructed wetlands and biofilter-treated surface water (Feng et al. 2013; Ansola et al. 2014). This may be because Proteobacteria includes a variety of bacteria contributing to the carbon and nitrogen cycle, and these bacteria are involved in the biodegradation or biotransformation of organic compounds (Liu et al. 2014; Santos et al. 2019).
In this study, four different substrates were added to the surface of the sediments to investigate their effects on the growth of submerged plant V. natans and rhizosphere microorganisms. The results showed that the addition of substrates to the sediment increased leave number, root growth, and root activity, thus promoting the growth of plants. The addition of maifanite had the most significant promotion effect on photosynthesis and plant growth. Under adverse transplantation conditions, the maifanite addition group exhibited the highest mean SOD activity level, and drastically decreased MDA content, and thus this group had a strong adaptability to the environmental changes. The increased relative abundance ofDesulfobacterota and Nitrospirota in rhizosphere sediments of substrate addition groups indicated that the addition of substrates had promoted the conversion of nitrogen sulfide in the sediments. Overall, our results indicated that the substrate addition, especially maifanite addition, could promote the growth of submerged plants and improve the rhizosphere microbial community structure on the sediment surface. Our findings provide valuable reference for screening sediment substrate improver.